Limits of mass transfer enhancement in lithium bromide-water absorbers by active techniques

Abstract

A model to predict the theoretical limits of mass transfer enhancement in a falling film absorber using LiBr aqueous solution has been developed and it provides a means of comparative absorber performance evaluation. During vapor absorption, the vapor- liquid interface becomes saturated immediately after it is exposed to vapor. Further absorption can only be sustained by mass diffusion and heat conduction into the fluid bulk, assuming a stationary interface. Due to the relatively small heat and mass diffusivities of LiBr aqueous solution, the mass absorption rates sustained by pure diffusion and conduction processes are small. To increase the mass absorption rates, mixing of the solution is favored. Therefore, every mass transfer enhancement technique, passive or active, consists of basically disturbing the film and causing mixing of the solution near the interface. The higher the mixing rate, the higher the mass absorption rate. To assess the effect of a mixed interface, a mathematical model is formulated. This model solves one-dimensional heat and mass differential equations coupled at the interface. An asymptote of the mass transfer rate as the mixing rate increases is derived. The model, which takes into account all relevant parameters such as temperatures, pressures, concentrations, mass flow rates, and geometry, is an ideal tool for rating absorber performance. Results show that, under typical operating conditions found in commercial chillers, the theoretically possible maximum mass absorption rate is 0.049 kg m 2 · s −1, and that, at a mechanically feasible mixing frequency of 1000 Hz, a mass absorption rate of 0.0256 kg m 2·s −1 is possible. The latter rate is about an order of magnitude larger than that found in commercial chillers.